1. Field of the Invention
The present invention generally relates to a telephone. More specifically, the present invention is directed to a telephone capable of being increasing the call connection efficiency in the telephone system.
2. Description of the Prior Art
Very recently, rapid digital technology has been realized in the mobile communication system, so that the analog type mobile communication system could be replaced by the digital type mobile communication system. Normally, the control signal should be transmitted and received between the radio base station connected to the communication line network and the portable telephone in the mobile communication system. When the digital signal is utilized as this control signal, it is possible to establish such a basic system capable of realizing radio signal transmissions at a high efficiency and also high communication quality. Moreover, utilization of the digital technique in the mobile communication system could make compact radio base stations as well as compact portable telephone, resulting in easy portability of the portable telephone.
Under such circumstances, a substantial amount of digital portable telephones have been rapidly marketed. This is because various problems of the conventional analog type portable telephone system could be effectively solved. That is, since the usable frequency allocation in the analog type portable telephone system is restricted, it is not so easy to increase the total number of telephone subscribers. Furthermore, the privacy of telephone conversations could not be satisfactorily protected. However, these problems belonging to the analog type portable telephone system can be successfully solved by the digital type portable telephone system.
On the other hand, a so-called "second generation cordless telephone system" in which the digital technology is comprehensively utilized is practically studied. A radio telephone utilized in this second generation cordless telephone system has such a feature that this radio telephone may be used as the original portable telephone and also a cordless telephone of a mother telephone.
In other words, the radio telephone used in the second generation cordless telephone system separately establishes one communication channel with the public radio base station connected to the communication line network, and another communication channel with the private (customer) radio base station corresponding to the own mother telephone. Also, the communication areas capable of establishing the independent communication channels between the respective base stations and the own telephone. However, there is a limitation in using the cordless (radio) telephone belonging to the private radio base station, although any radio telephones can be freely used with the public radio base station. When the dialed (called) radio telephone is used as the cordless telephone, the telephone calling person needs to dial the customer telephone number (private telephone number) of the radio telephone. When the called radio telephone is used as the portable telephone, the telephone calling person needs to dial the telephone number (public telephone number) of the radio telephone itself.
In this second generation cordless telephone system, the communication areas of the radio base stations are limited to the small zones referred to as "microcells". In these microcells, the same frequencies are repeatedly utilized to make up a large number of communication channels. In other words, the limited frequency resource can be effectively utilized. Furthermore, since the dimension of the communication zone (cell) is restricted to a small value, the transmission power of the radio telephone may be made low, so that a compact radio telephone with low power consumption can be made available. That is, the physical size of the batteries mounted on this radio telephone becomes small, and a long operation time of the radio telephone can be achieved.
However, the above-described second generation cordless telephone system has the following drawbacks. That is, while an available range of communication for one radio base station is limited to several hundred meters (radial direction), the same frequencies are usable in the different communication zones at the same time so as to effectively allocate all the usable radio frequencies. To this end, the radio base stations must be installed at 100 meter intervals. Thus there is such a problem that the radio telephone should be continuously located near the relevant radio station, e.g., shorter than 100 meters. When the opposite (called) radio telephone is used as the cordless telephone of the mother telephone, and even when the telephone calling person dials the public telephone number of this called radio telephone, he cannot access this called radio telephone. On the other hand, when the opposite radio telephone is utilized as the portable telephone, there is another problem that even if the telephone calling person dials the private telephone number of this called radio telephone, then he cannot access this called radio telephone.
There are some possibilities in the second generation cordless telephone system that the radio telephone cannot establish the communication in case that the radio telephone is located outside the available range of communication covered by the public radio base station, is not set to the telephone waiting condition, or the relevant communication channel is fully occupied by other radio telephones.
One solution has been proposed as the radio telephone equipped with the paging function. That is, the radio telephone is combined with a pager with low power consumption operable in the paging system which can cover a wider communication range than that of the second generation cordless telephone system. That is, such a wide communication range can be established by utilizing the radio telephone with the paging function in such a manner that the pager communication can be made with another radio telephone equipped with the paging function located outside the available range of communication in the second generation cordless telephone system.
However, when no telephone line is connected in case that the user issues the telephone calling to the opposite (called) radio telephone with the paging function by utilizing the second generation cordless telephone system, he should make another telephone calling with using the paging system. Under such circumstances, there is a drawback that a desired telephone number is selected from a plurality of stored telephone numbers, or the relevant telephone number should be inputted so as to make a telephone call.